1. Field of the Invention
This invention generally relates to a printer having a thermal ink jet printhead, and more particularly to a thermal ink jet printer printhead having a switch stand-by mode for reducing the power consumption and the temperature of the printhead during stand-by periods of the printer.
2. Description of Related Art
A known thermal ink jet printer uses a compact and reliable microelectronic printhead. The printhead contains four essential elements: robust static voltage spike protection, logic addressing circuitry, power MOS drivers, and heater elements for heating the ink. These essential elements are formed on a microelectronic heater die formed from a silicon wafer.
The logic addressing circuitry is used to control the firing of the heater elements by turning on and off the power MOS drivers. Predriver circuitry is used to boost the five volt logic level signals output by the logic addressing circuitry to voltage levels sufficient to drive the power MOS drivers.
In the conventional thermal ink jet printer, the intermediate power supply to the predriver circuitry is left permanently on, regardless of whether the printer is in a printing operation or in a stand-by mode between printing operations. The power supply continuously supplies power to the predriver circuits even when the printer is in a stand-by mode even though they are not actively driving the heater elements. This continuous power draw of the predriver circuitry in the stand-by mode is undesirable, both because it needlessly wastes power and because the needlessly wasted power is converted to heat, eventually overheating the printhead.
When the printhead overheats, various Droblems result. First, the resolution of the printer depends on the spot size of an ejected droplet of ink. Nominally, the spot size it about 130 .mu.m. The spot size depends upon the viscosity of the ink. In turn, the viscosity of the ink depends upon the temperature of the ink, which is approximately equal to the temperature of the printhead. Thus, when the printhead temperature increases, the viscosity of the ink decreases and the spot size of the resulting ink droplet increases. This increased spot size reduces the optical quality of printing.
Further, ink jet printers generate many colors by mixing together droplets of a number of different colors of ink ejected from different printhead heater elements. Thus, the increased printhead operating temperature reduces the color stability by changing the proportion of inks mixed together to form a particular color.
Second, when the temperature of the printhead rises above a critical temperature (the ingestion temperature) the printhead takes in air and then is unable to operate until reprimed. Therefore, the image quality of the image formed degrades and soft failure modes occur.
Third, when the printhead temperature increases during a stand-by or non-printing mode, the volatile constituents (generally, water) of the liquid thermal ink evaporate from the printhead and condense on the cooler surrounding surfaces. Thus not only does the increased printhead temperature reduce the reliability of the printhead through drying out of the ink, but the surrounding surfaces become contaminated with the volatiles evaporated from the ink.